1. Field of the Invention
The present invention generally relates to heart conditioning, and in particular to ventricular tachyarrhythmia detection allowing a discrimination between stable and unstable tachyarrhythmia.
2. Description of the Prior Art
Implantable medical devices (IMDs), including implantable cardiac defibrillators and pacemakers, can today be used for detecting and combating ventricular tachyarrhythmia in IMD patients. Ventricular tachyarrhythmias, for example ventricular fibrillation or tachycardia, need to be detected as early as possible as they may otherwise lead to the death of the patient if not quickly terminated. As a consequence, once tachyarrhythmia is detected, the IMD will combat it by delivering one or more defibrillation or cardioversion shocks.
Such defibrillation shocks are very uncomfortable to the patient and the risks of inappropriate and unnecessary shock applications should therefore be minimized.
Today different techniques can be used for detecting a ventricular fibrillation. A typical example is to measure the intervals between ventricular depolarization, i.e. ventricular cycle length (VCL). However with the techniques employed today, tachyarrhythmia may be detected and shocks may be applied even though the tachyarrhythmia is stable, temporary and would pass off by itself. Thus, detection techniques of today are not able to discriminate between unstable tachyarrhythmias that indeed should be combated by defibrillation shocks and other stable tachyarrhythmias that do not need any shock therapy.
U.S. Pat. No. 5,311,874 discloses a method for tachycardia discrimination. In a first embodiment, a cardiac biopotential signal is recorded and processed to identify a number of feature values representing maximum and minimum values of a complex in the signal, corresponding to a complete cardiac cycle. Firstly, the complex is classified as a baseline complex or a non-baseline complex based on the cycle length of the complex. If the complex is classified as a non-baseline complex, an extensive and very complex processing of its feature values is conducted to subsequently arrive at a discrimination point in a plane defined by a similarity vector and a dissimilarity vector. Depending on where this point is in the plane, the non-baseline complex is classified as a ventricular tachycardia (VT) or non-VT complex. In a second embodiment, a corresponding complex signal processing is performed but for discriminating between hemodynamically stable and unstable ventricular tachycardias. In this case, the input signal can be a signal or condition related to the hemodynamics of the heart, such as pressure, flow or impedance.
United States Published Patent Application No. 2005/0154421 discloses a technique for reducing inappropriate delivery of therapy to treat ventricular tachyarrhythmias caused by supraventricular tachycardia (SVT). The document specifies that SVT can be conducted to the ventricles and lead to short VCLs that would imply ventricular tachyarrhythmia. Their technique is based on measuring multiple VCLs over a defined time period. The number of such cycles that have a length shorter than a given threshold is determined and used as a basis for detecting ventricular tachyarrhythmia. If tachyarrhythmia is detected, it is determined whether the tachyarrhythmia is due to SVT or may indeed be lethal. This determination can be based on measured VCLs and atrial cycle lengths (ACLs), measured activity level of the patient or intracardiac pressure measurements.
If it is determined that the tachyarrhythmia is due to SVT no therapy or a modified form of therapy is applied, while otherwise the patient will be shocked by the IMD.
However, SVTs may indeed cause unstable ventricular tachyarrhythmias that are lethal to the patient and should be treated by defibrillation shocks. Thus, the discrimination between application of shocks or no shocks based on whether the tachyarrhythmia originates from SVTs, as determined by the document US 2005/0154421, may lead to incorrect decisions whether the patient should be shocked.
Furthermore, the technique disclosed in U.S. Pat. No. 5,311,874 is marred by the disadvantage of requiring extensive signal processing that will quickly drain the power supply when implemented in an IMD and also occupy a substantive portion of the processing capacity of the IMD. Furthermore, the document uses the same signal for classifying a signal complex as baseline or non-baseline as for classifying a non-baseline complex as hemodynamically stable or unstable which is not optimally.